[Discuss] Default mappings for dynamic fields

Today all the templates are setup with the following dynamic templates

```
"dynamic_templates" : [
{
"strings_as_keyword" : {
"mapping" : {
"ignore_above" : 1024,
"type" : "keyword"
},
"match_mapping_type" : "string"
}
}
],
```

By default all the unknown fields are mapped by Elasticsearch and all matching string to keyword. This issue is to discuss what our defaults should be moving forward.

Mapping all fields by default comes at a storage cost. In recent releases Elasticsearch has introduced runtime fields and `dynamic: runtime` as an option. An other option is to disable indexing completely and only allow runtime fields on query time.

Related issue: https://github.com/elastic/package-spec/issues/39